1. Field of the Invention
The invention generally relates to DC (Direct Current) motors used in various applications, such as hard disk drive motors, cooling fans, drive motors for appliances, etc.
2. Description of the Related Art
Electric motors are used to produce mechanical energy from electrical energy, used in a number of applications, including different household appliances, pumps, cooling fans, etc. Electric motors are generally classified as either alternating current (AC) motors or direct current (DC) motors.
Motors generally include a rotor, which is the non-stationary (moving) part of the motor, and a stator, which is the stationary part of the motor. The stator generally operates as a field magnet (e.g., electromagnet), interacting with an armature to induce motion in the rotor. The wires and magnetic field of the motor (typically in the stator) are arranged so that a torque is developed about the rotor's axis, causing rotation of the rotor. A motor typically also includes a commutator, which is an electrical switch that periodically reverses the current direction in the electric motor, helping to induce motion in the rotor. The armature carries current in the motor and is generally oriented normal to the magnetic field and the torque being generated. The purpose of the armature is to carry current crossing the magnetic field, thus creating shaft torque in the motor and to generate an electromotive force (EMF).
In a typical brushed DC motor, the rotor comprises one or more coils of wire wound around a shaft. Brushes are used to make mechanical contact with a set of electrical contacts (called the commutator) on the rotor, forming an electrical circuit between the DC electrical source and the armature coil-windings. As the armature rotates on an axis, the stationary brushes come into contact with different sections of the rotating commutator. The commutator and brush system form a set of electrical switches, each firing in sequence, such that electrical-power always flows through the armature coil closest to the stationary stator (permanent magnet). Thus an electrical power source is connected to the rotor coil, causing current to flow and producing electromagnetism. Brushes are used to press against the commutator on the rotor and provide current to the rotating shaft. The commutator causes the current in the coils to be switched as the rotor turns, keeping the magnetic poles of the rotor from ever fully aligning with the magnetic poles of the stator field, hence maintaining the rotation of the rotor. The use of brushes creates friction in the motor and leads to maintenance issues and reduced efficiency.
In a brushless DC motor, the commutator/brush-gear-assembly (which is effectively a mechanical “rotating switch”) is replaced by an external electronic switch that's synchronized to the rotor's position. Brushless DC motors thus have an electronically controlled commutation system, instead of a mechanical commutation system based on brushes. In a brushless DC motor, the electromagnets do not move, but rather the permanent magnets rotate and the armature remains static. This avoids the problem of having to transfer current to the moving armature. Brushless DC motors offer a number of advantages over DC motors featuring brushes, including higher efficiency and reliability, reduced noise, longer lifetime (no brush erosion), the elimination of ionizing sparks from the commutator, and overall reduction of electromagnetic interference (EMI).
One issue oftentimes taken into consideration when designing motors, more specifically brushless motors, is the power required to operate the motor. One technique to reduce power in some applications has been the introduction of Three Phase Brushless DC (TPDC) Motors. Another method makes use of six (6) drive transistors. Prior art literature describes many different techniques for both basic commutation and enhanced techniques designed to improve the acoustic signature of the fan under operation. The architectures for these solutions fall into two basic categories, Mixed-Signal Micro-Controller Unit (MCU), or analog Application Specific Integrated Circuit (ASIC). The simplest of all these is the low-side commutation scheme.
There are a number of Analog ASIC solutions using differential commutation schemes currently available. The range of complexity in the algorithm varies, with attempts to “soften” the startup current through various techniques, until the Back Electro-Motive Force (BEMF) signal can be detected. The Analog ASIC solutions tend to use external components to generate reference ramps and saw-tooth waveforms to be used as references against motor coil responses. While more compact, the analog solutions rely on external passive components to control the operational set points in the IC.
The Mixed-Signal MCU typically comprises an 8051-based micro-controller with various Analog-to-Digital Converter (ADC) modules, comparators, and in some cases filter modules. The MCU accurately controls timing, performs complex calculations and transformations, and enables communications directly with an external controller. High voltage solutions separate the electronics using either of these approaches and use external transistors and isolation.
In order to control the speed of the motor through a given command, either a voltage or PWM duty cycle, an error signal is developed. The theoretical method is to measure slope of the BEMF signal as the rotor passes the stator coil and use that information to determine the position of the rotor. A BEMF signal that is offset from its midpoint is typically indicative of the rotor deviating from the electrical commutation. If the BEMF signal is too high and early, it is an indication of the rotor spinning faster than the electrical commutation, and the next commutation period must be lengthened. Likewise, a BEMF signal that is too low and late would be indicative of the rotor spinning slower than the electrical commutation, requiring shortening the period. Developing this type of error signal in digital circuitry has required a microcontroller or microprocessor in the past, as well as a high speed Analog-to-Digital converter (ADC). One alternative method includes the development of analog circuitry to generate reference pulse trains, and the use of analog components to phase lock to the BEMF signal.
While these solutions have successfully provided some power savings, in many instances such power savings fell short of expectations. Since most, if not all of these solutions are designed for a specific motor type, they cannot easily be ported from application to application, or even from manufacturer to manufacturer. Each motor type requires tuning capacitors to adjust the commutation and startup frequencies, as well as crossover and dead-time locations in the commutation sequence.
One of the major disadvantages of prior art solutions has been the lack of power savings realized. All literature discusses power savings in the range of 15-30% over other solutions, with as much as 50% in the mid-range of the motor being driven. While some techniques operate to reduce the acoustic noise produced when switching the stationary electromagnets using PWM methods, additional power is yet required to drive the motor coils in this manner. The idea is to reduce the overall inductive spikes caused when the drive transistors are turned off. The literature contends switching the coils at a rate much, much higher than the commutation frequency will “soften” the switching and reduce the acoustic signature. Depending on the amount of time needed to “soften” both the rising and falling edges, as much as 30% of the overall time finds all three drive transistors conducting, increasing the current consumption by ⅓, since all 3 coils are conducting.
Another source of power inefficiency in some prior art solutions is the lack of “differential commutation”. In other words, both the high-side and low-side transistors are switched off simultaneously, as a method to control the rotational speed. In a classic configuration, the high-side of the motor is connected directly to the power source.
Some implementations do not control the frequency or duty cycle of the PWM signals going to the drive transistors, but rather allow the incoming PWM to modulate the signals directly. The inability to limit either frequency or duty cycle means the motor is not being driven optimally for a given operating point, but is under the control of an external device that may or may not be aware of the motor limitations. This will cause the motor to use more current than required, producing additional heat that must be removed from the system.
Prior art solutions also fail to address the issue of over-current/lock rotor. The currents used are sufficient to damage the motor windings, and without a feedback method, a timer must expire before the damaging condition can be detected and corrected. If there is no provision for this event, the motor will continue to drive to destruction. Many solutions use a brute-force method to drive the motor coils during start-up, and may last several seconds, drawing several times the normal operating current. The period of time when this occurs is commonly referred to as the Forced Commutation phase of spin-up, and it is one of the drawbacks of the BEMF commutation method. Until the motor spins sufficiently fast enough to generate a BEMF signal, the motor is driven in an open loop configuration, at a predetermined frequency and PWM duty cycle, putting undue stress on the motor components.
Another issue present with respect to TPDC motors has been the presence of mechanical vibrations and altering the inherent acoustic signature of the fan impeller. TPDC motors may be driven with either sinusoidal or trapezoidal current waveforms. Classic drive schemes rely on creating zero current switching in order to minimize the effects of changes in instantaneous torque. These periodic changes in torque occur when the coils are energized and de-energized, or commutated, producing both mechanical vibration, and altering the inherent acoustic signature of the fan impeller. Both effects are undesirable in many applications, including fans for the PC industry.
Therefore, improvements in motor design and operation are desired.